Various Modifying Agencies. 137 



partly formed when the eggs are laid ; and it is possible to compel a 

 snake to become viviparous artificially. This has been done by M. 

 Geoffroy by depriving a common snake of water. In this case the eggs 

 could not be laid by the snake but were retained until they were 

 hatched internally. The development of the lizzard, Axolotl, into its 

 adult and final form, called the Amblystoma, is effected by depriving 

 the former of water. 



There are over twent} T species of Amblystomas in America, some of 

 which in their development stop at the Axolotl stage, while others go 

 on to become Amblystomas. The difference between the two is much 

 the same as between their Amphibian relations, the tadpole and the 

 frog. Like the tadpole the Axolotl breathes in the water by means of 

 gills, and like it acquires lungs, but unlike it does not always drop the 

 gills and pass into an air-breather exclusively. As an Axolotl, with 

 both gills and lungs, it may live, reproduce Axolotls and die without 

 further development. But according to experiments with them in sev- 

 eral places in Europe, when the Axolotls are about six months old their 

 lungs are so far developed that they can almost live without using their 

 gills at all. If they are then placed where they can choose between 

 land and water, they will spend part of the time on land, and by grad- 

 ually diminishing the water till it is all gone, their gills will become 

 dessicated and shrivel, and the whole of the respiration then becomes 

 aerial. In eight days after finally leaving the water, their gill slits are 

 entirely obliterated, the skin is moulted, the dorsal crest or fin is shriv- 

 elled up, the tail becomes more rounded, and the adult form of the 

 Amblystoma is reached, the whole process taking place within nine 

 months of birth. Undoubtedly this is precisely the process by which 

 this curious development is brought about in a state of nature. These 

 animals inhabit ponds that, in the drouthy districts of Mexico and 

 Western United States, are apt to dry up in summer, and they have, by 

 hereditary habit, gradually come to adopt aerial breathing and to rely 

 upon it exclusively when necessary. But we may conclude that the 

 habit is of comparatively recent origin, because the gills require to be 

 mechanically shrivelled by the dry air, and if the animal has plenty of 

 water, he may remain in it and not shed his gills at all. In the case of 

 the Frog, the habit has become a necessary link in the chain of its devel- 

 opment, and the gills are shed at a certain stage regardless of moisture. 



But there is another case arising from the same cause that has gone 

 further. 



The black Salamander of the Alps, is exclusively a lung breather, 

 and does not inhabit the water at all. Nevertheless, the embryos, which 

 are retained within the body of the female to be hatched and developed, 

 have complete gills which they shed before they are born. If these 



